The invention pertains to polyester compositions and the preforms and bottles molded from the compositions, and more particularly to polyester compositions, preforms, and bottles having a reduced coefficient of friction while maintaining low haze.
Problems exist in handling and conveying various types of polyester containers due to the excessive amount of static friction encountered when container surfaces contact each other. The following is a brief description of areas in container packaging industry where problems have been encountered due to excessive coefficient of static friction (COF).
During the process of injection molding preforms, the preforms are often immediately fed into a large box (termed gaylord box) which can hold  greater than 1000 preforms. With the high coefficient of friction that is common between PET surfaces, the preforms tend to stack on top of one another in a conical shape (as viewed from the side of the box) instead of sliding past one another and giving more of a flattened out appearance, as would a liquid being poured into a container. As a result, fewer preforms are loaded into a box that leads to higher shipping costs per preform.
The next step for preform processing is the transferal of the preforms from the box into a stretch-blow molding machine feeder bin. In the presence of a high level of friction between preform surfaces, jams can occur in the feeder bin as preforms are loaded onto the feed rail. Feed rail jams can also occur due to this high level of friction between the preform surfaces.
During the process of blowing and filling stretch blow-molded PET, CSD bottles, it is common to convey bottles along conveyor belts or rails, e.g., the moving of bottles from the stretch blow-molding machines to a palletizer area, or depalletizing and moving bottles through a labeling and filling process. At certain points in this process, usually at the palletizing and de-palletizing areas, several rows of bottles may be merged into one row for filling, labeling, palletizing, etc. At this point the pressure between the bottle surfaces is increased. Surfaces that exhibit high COF will resist sliding at the point of bottle merger and lead to bottle sticking and cause line disruption.
Certain bottle types, such as the popular 2-Liter bottles, are straight-walled and have very smooth surfaces obtained from the highly polished mold. Although smooth surfaces provide an appealing appearance, the flat surface maximizes the area of contact between bottles. When PET displays a high coefficient of friction (i.e. static coefficient of friction greater than 1.0), bottles become entangled so as to xe2x80x98tip overxe2x80x99 or just stop moving in the conveying line, causing disruptions in the process, which are very undesirable. The high coefficient of friction prevents adjacent bottles on a multiple-row conveying line from moving (turning or slipping) during conveying. When the conveying line changes direction, sometimes as much as 90 degrees, bottles become entangled and, either stay upright and stop the feed, or, bottles fall over and stop the progression of the line. In either event, someone has to be near these problem areas at all times in order to maintain the progression.
Thus, a method to produce a preform and bottle containing a low static coefficient of friction, which would allow the preforms to slide past one another and allow bottles to slide and rotate against other bottles during this conveying stage of the xe2x80x98stretch-blowxe2x80x99 and xe2x80x98filling plantxe2x80x99 processes, would minimize or eliminate downtime and also the need for someone to constantly monitor the situation.
We have found that silicon carbide imparts to a polyester resin a low coefficient of static friction.
Thus, in one embodiment, there is provided a polyester pellet composition, a preform, and a bottle, each comprising a polyester polymer and silicon carbide.
Preferably, each contain from 5 ppm to 1000 ppm silicon carbide. We have also discovered that a bottle having both a low bottle sidewall haze and a low coefficient of static friction can be made from compositions containing silicon carbide.
In yet other embodiments, there is also provided a process for manufacturing a polyester composition, comprising adding a solid or liquid concentrate comprising silicon carbide and polyethylene terephthalate to bulk polyethylene terephthalate after melt phase polymerization of the bulk polyethylene terephthalate and before or at injection molding the polyester composition.
In yet another embodiment of the invention, there is provided a process for manufacturing a polyester composition, comprising adding silicon carbide neat or as a concentrate or in a carrier to a melt phase for the manufacture of polyethylene terephthalate.
In another embodiment, there is provided a process for manufacturing a polyester composition, comprising adding silicon carbide to a melt phase during the polymerization of polyethylene terephthalate and feeding the molten polyethylene terephthalate to an injection molding machine for the manufacture of a preform.
In a further embodiment, there is provided an isolated concentrate composition comprising silicon carbide in an amount ranging from 0.05 wt. % to about 35 wt. % and a thermoplastic polymer, preferably polyethylene terephthalate, in an amount ranging from at least 65 wt. % up to 99.95 wt. %, each based on the weight of the concentrate composition.
There is also provided a preform and the bottle made from the preform comprising silicon carbide having an L* rating of 75.0 or more, and bottle sidewall haze level of 4.0% or less.
There is also provided a polyester composition, and a preform, and a bottle made from the preform, having a bottle sidewall haze value of 4.0% or less, and a coefficient of static friction of 0.8 or less.